What makes the colors different?
The two cobalt salts pictured above both contain Co2+ cations. The difference in color is due to the species surrounding the cobalt ion. The presence of water molecules in the coordination sphere around the central cobalt ion changes the distances among species and the color of the material.
The coordination number is the number of ions that immediately surround an ion of the opposite charge within a crystal lattice. If you examine the figure below, you will see that there are six chloride ions immediately surrounding a single sodium ion. The coordination number of sodium is 6. Likewise, six sodium ions immediately surround each chloride ions, making the coordination number of chloride also equal to 6. Because the formula unit of sodium chloride displays a 1:1 ratio between the ions, the coordination numbers must be the same.
The formula unit for cesium chloride is CsCl, also a 1:1 ratio. However, as shown in the figure below, the coordination numbers are not 6 as in NaCl. The center ion is the Cs+ ion and is surrounded by the eight Cl− ions at the corners of the cube. Each Cl− ion is also surrounded by eight Cs+ ions. The coordination numbers in this type of crystal are both 8. CsCl and NaCl do not adopt identical crystal packing arrangements because the Cs+ ion is considerably larger than the Na+ ion.
Another type of crystal is illustrated by titanium(IV) oxide, TiO2, which is commonly known as rutile. The rutile crystal is shown below.
The gray Ti4+ ions are surrounded by six red O2− ions. The O2− ions are surrounded by three Ti4+ ions. The coordination of the titanium(IV) cation is 6, which is twice the coordination number of the oxide anion, which is 3. This fits with the formula unit of TiO2, since there are twice as many O2− ions as Ti4+ ions.
The crystal structure of all ionic compounds must reflect the formula unit. In a crystal of iron(III) chloride, FeCl3, there are three times as many chloride ions as iron(III) ions.
- The coordination number of a compound is determined by the type and number of ions or other species surrounding a central ion.
- Often the color of a compound is affected by the specific materials coordinated to that central ion.
- What is the coordination number for Na+ in NaCl?
- What is the coordination number for Cs+?
- Why are the packing arrangements for Na+ and Cs+ different?